Treatment of carbonylation residues

ABSTRACT

Residues are formed in the carbonylation of esters or ethers, particularly in the production of acetic anhydride or ethylidene diacetate. When such residues contain noble metal, typically rhodium, used as a catalyst, the noble metals must be recovered before the residues can be disposed of. The metal values are freed from the residues by treatment with amines, thereby enabling the rhodium to be extracted by subsequent contact with an aqueous halogen acid. The residues are pretreated with alkanols and concentrated by evaporation to improve the effect of such amine treatments.

PRIOR ART

Complex catalysts employing Group VIII noble metals, particularlyrhodium, are used for the homogenous catalysis of reactions in whichcarbon monoxide and hydrogen may be reacted with various organicmolecules to produce compounds having a higher molecular weight. Thereactions of particular interest with respect to the present inventionare those generally designated as hydroformylation and carbonylation.Such reactions are shown in many patents, for example U.S. Pat. Nos.3,579,552 and 4,115,444 and British Pat. Nos. 1,468,940 and 1,538,782.The noble metal catalysts are considered to be complexes which typicallyinclude carbon monoxide, promoting metals, and other non-metallicpromoters, particularly phosphorus-containing ligands.

Reaction products must be separated from the homogenous catalyst.Typically, this is done by distilling the reactor effluent to separatethe organic compounds and leaving behind the noble metal catalyst andother heavier materials which can then be recycled to the reactionvessel. The prior art discloses means by which the noble metal isrecovered directly from reactor effluents for further use. However, ingeneral the art indicates that heavy residues accumulate and must bepurged from the reaction system. Such residues contain substantialamounts of noble metal which must be recovered in order for the processto be carried out economically. Since rhodium is the principal noblemetal used, the discussion herein will refer for conveniencespecifically to rhodium, but it is to be understood that other noblemetals are not excluded.

Rhodium has been recovered by many techniques, but at least threegeneral approaches have been disclosed. First, the rhodium is recoveredas the metal itself, which could require reformulation of the catalystfor further use. Second, the rhodium may be recovered on a solidmaterial, which may serve as a catalyst support. Third, the rhodium isrecovered in a form acceptable for returning to the reactor, with orwithout some additional processing to improve its catalytic properties.

Rhodium may be recovered as a metal by pyrolysis as shown in U.S. Pat.No. 3,920,449, which involves the high temperature decomposition ofresidues and rhodium-containing catalyst. The rhodium can then bereprocessed as required to provide catalyst or catalyst precursors forrecycle to the reaction mixture.

The second recovery technique may be illustrated by U.S. Pat. No.3,899,442 in which rhodium is deposited on a solid support inconjunction with pyrolysis of the residues. An alternative is shown inU.S. Pat. No. 3,978,148 in which rhodium is adsorbed on activatedcarbon, from which it could be recovered.

The third recovery technique is of particular interest with respect tothe present invention, since it involves a recovery of rhodium byprecipitation from solution in a form which is not necessarily metallic,but may be returned directly to the reaction vessel or pretreated beforethe recycling. Many methods of this sort have been disclosed in theprior art. Although the catalyst typically is soluble under reactionconditions, it may be possible to form insoluble compounds by theaddition of water as shown in U.S. Pat. Nos. 2,839,580, 2,880,241, and3,821,311. U.S. Pat. No. 3,887,489 shows the precipitation ofrhodium-containing compounds by heating reaction residues for asufficient period of time and temperature. Another technique shown inU.S. Pat. No. 3,560,359 is the use of hydrogen or hydrides to reducecarbonyl content of the tar to hydroxyl groups and thereby to releasethe rhodium complex which precipitates and can be recovered. Theselective adsorption of the residues to separate them from the rhodiumcatalyst is in U.S. Pat. No. 3,539,634. The opposite approach, namely,the adsorption of rhodium on a solid adsorbent is disclosed in U.S. Pat.No. 3,716,626.

Particularly relevant to the present invention is prior art showing theextraction of rhodium from reaction residues with strong acidsaccompanied by water and often solvents. Typical disclosures are foundin U.S. Pat. Nos. 3,420,873, 3,641,076, and 3,857,895. Relatedtreatments with acids and peroxides are shown in U.S. Pat. Nos.3,547,964 and 4,021,463.

In connection with the carbonylation process to be completely describedboth hereinafter and in U.S. Pat. Nos. 4,115,444 and 4,251,458 andBritish Pat. Nos. 1,468,940 and 1,538,782, it has been found thatcarbonylation reaction residues are not easily separated from therhodium which they contain. Use of acid treatments typical of the priorart have been found to provide only incomplete recovery of the rhodiumcontent. Since the remaining rhodium appears bound to the reactionresidues, an improved method for completely recovering those rhodiumvalues has been sought and is disclosed in commonly assignedapplications. It has been found that pretreatment of such residuesaffects the ability to free the rhodium content as disclosed hereafter.

SUMMARY OF THE INVENTION

Residues created during carbonylation reactions, particularly thecarbonylation of esters or ethers, especially carbonylation of methylacetate or dimethyl ether to acetic anhydride or ethylidene diacetate,appear to bind Group VIII noble metals typically rhodium, and to beresistant to extraction by strong acids. When such residues are treatedwith amines, preferably primary aliphatic amines and/or hydrazine, therhodium can then be extracted by subsequent contact with a halogen acid.Preferred are compounds containing 0-4 carbon atoms, especially n-propylamine and hydrazine.

The ability of the amines to free rhodium from the residues is affectedby the manner in which the volatile components are removed from theresidues. According to the method of the invention, residues are treatedwith alkanols, preferably methanol, and concentrated by heating atrelatively low temperatures, preferably no higher than 25° C. under asuitable vacuum, typically about 25 mm Hg absolute, until substantiallyall of the volatile components have been removed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The recovery of Group VIII noble metals, especially rhodium, fromcarbonylation and hydroformulation reaction residues has been ofconsiderable interest to those skilled in the art. Of particular concernto the present inventors is the recovery of Group VIII noble metals,particularly rhodium, from catalysts used in the carbonylation of acarboxylate ester or an alkyl ether to an anhydride, especially thecarbonylation of methyl acetate or dimethyl ether to acetic anhydride.In another aspect, the invention relates to recovery of similar rhodiumcatalysts used for the carbonylation in the presence of hydrogen ofmethyl acetate or dimethyl ether to ethylidene diacetate. Theseprocesses have been described in depth in U.S. Pat. Nos. 4,115,444 and4,251,458 and in British Pat. Nos. 1,468,940 and 1,538,782 and they aresummarized below. A related process is shown in British publication GBPat. No. 2,038,829A in which methyl acetate is reacted with carbonmonoxide and hydrogen to produce acetaldehyde in the presence of apalladium catalyst. The processes are important since they producechemicals of value, both for direct use and as intermediates. However,the recovery of Group VIII noble metals according to the presentinvention is not considered to be limited only to these particularcarbonylation processes.

Preparation of Carboxylic Acid Anhydrides

The process for the preparation of an anhydride of a monocarboxylic acidin general comprises carbonylation of a carboxylate ester (RCOOR) or anether (ROR) in the presence of a Group VIII noble metal catalyst and ahalogen, the R's may be the same or different and each R is a monovalenthydrocarbon radical or a substituted monovalent hydrocarbon radicalwherein any substituent is inert.

Of particular interest, acetic anhydride can be effectively prepared bycarbonylating methyl acetate or dimethyl ether under a moderate COpartial pressures in the presence of a Group VIII noble metal catalystand iodides or bromides. Various metallic and non-metallic promoters maybe present. Other alkanoic anhydrides, such as propionic anhydride,butyric anhydrides and valeric anhydrides, can be produced bycarbonylating the corresponding lower alkyl alkanoate or a lower alkylether.

Preparation of Ethylidene Diacetate

The preparation of ethylidene diacetate comprises contacting (a) methylacetate or dimethyl ether, (b) carbon monoxide, and (c) hydrogen with asource of halide comprising a bromide and/or an iodide, within areaction zone under substantially anhydrous conditions in the presenceof a Group VIII noble metal catalyst. Again, various metallic andnon-metallic promoters may be present.

The overall reaction can be expressed by the following chemicalequation:

    2 methyl acetate+2CO+H.sub.2 →ethylidene diacetate+acetic acid

When dimethyl ether is used as the reactant in lieu of methyl acetate,the overall reaction is can be expressed by the following chemicalequation:

    2 dimethyl ether+4CO+H.sub.2 →ethylidene diacetate+acetic acid

When using dimethyl ether as the organic raw material it is believedthat the initial step involved is the carbonylation of the ether toproduce methyl acetate. This may be done in a separate reaction zone.However, the use of a separate reaction zone is not necessary becausethe conversion of dimethyl ether to methyl acetate can be carried outconcurrently with and in the same reaction zone as that in which theethylidene diacetate is formed.

Reaction Conditions

In carrying out the reactions, a wide range of temperatures, e.g. 20° to500° C., are suitable but temperatures of 100° to 300° C. are typicallyemployed and the more preferred temperatures generally lie in the rangeof 125° to 250° C. The reaction is carried out under superatmosphericpressure and employing a carbon monoxide partial pressure which ispreferably 0.35 to 140.6 kg/cm², and most preferably 1.76 to 70.3kg/cm², although carbon monoxide partial pressures of 0.007 to 1055kg/cm² can also be employed. The total pressure is preferably thatrequired to maintain the liquid phase.

The Group VIII noble metal catalyst, i.e., iridium, osmium, platinum,palladium, rhodium or ruthenium, can be supplied in the zero valentstate or in any higher valent form. For example, the catalyst to beadded may be the metal itself in finely divided form, or as a metalcarbonate, oxide, hydroxide, bromide, iodide, chloride, lower alkoxide(methoxide), phenoxide or metal carboxylate wherein the carboxylate ionis derived from an alkanoic acid of 1 to 20 carbon atoms. Similarlycomplexes of the metals can be employed for example the metal carbonyls,such as iridium carbonyls and rhodium carbonyls, or as other complexessuch as the carbonyl halides, e.g., iridium tri-carbonyl chloride

    [Ir(CO).sub.3 Cl].sub.2

or the acetylacetonates, e.g. rhodium acetylacetonate

    Rh(C.sub.5 H.sub.7 O.sub.2).sub.3.

Preformed ligand-like complexes can also be employed, such as dichlorobis-(triphenylphosphine) palladium, dichloro bis-(triphenylphosphine)rhodium, and trichloro tris-pyridene rhodium. Other forms in which thecatalyst can be added to the system include, aside from those alreadyspecifically listed, rhodium oxide (Rh₂ O₃), tetrarhodiumdodecacarbonyl, dirhodium octacarbonyl, hexarhodium hexadecacarbonyl(Rh₆ (CO)₁₆), rhodium (II) formate, rhodium (II) acetate, rhodium (II)propionate, rhodium (II) butyrate, rhodium (II) valerate, rhodium (III)naphthenate, rhodium dicarbonyl acetylacetonate, rhodium trihydroxide,irdenylrhodium dicarbonyl, rhodium, dicarbonyl(1-phenylbutane-1,3-dione), tris(hexane-2,4-dione) rhodium (III),tris(heptane-2,4-dione) rhodium (III), tris(1-phenylbutane-1,3-dione)rhodium (III), tris(3-methyl-pentane-2,4-dione) rhodium (III), andtris(1-cyclohexylbutane-1,3-dione) rhodium (III).

The noble metal catalyst can be employed in forms initially oreventually soluble in the liquid phase reaction medium to provide ahomogenous catalyst system. Alternatively, insoluble (or only partiallysoluble) forms, providing a heterogeneous catalyst system, can beemployed. Amounts of carbonylation catalyst (calculated as containednoble metal based upon the total quantity of liquid phase reactionmedium) of as little as about 1×10⁻⁴ wt. % (1 ppm) are effective,although normally amounts of at least 10 ppm, desirably at least 25 ppm,and preferably at least 50 ppm would be employed. An optimum balancingof reaction rate and economic criteria would normally suggest the use ofamounts of contained noble metal carbonylation catalyst based upon thetotal weight of liquid phase reaction medium between about 10 and about50,000 ppm, desirably between about 100 and 25,000 ppm, and preferablybetween about 500 to 10,000 ppm.

Activity of the Group VIII noble metal catalysts described above can besignificantly improved, particularly with respect to reaction rate andproduct concentration, by the concurrent use of a promoter. Effectivepromoters include the elements having atomic weights greater than 5 ofGroups IA, IIA, IIIA, IVB, VB, VIB, the non-noble metals of Groups VIIIand the metals of the lanthanide and actinide groups of the PeriodicTable. Preferred inorganic promoters include the metals of Groups VIBand the non-noble metals of Group VIII, especially chromium, iron,cobalt, and nickel and most preferably chromium. Particularly preferredare the lower atomic weight metals of each of these groups, i.e. thosehaving atomic weights lower than 100, and especially preferred are themetals of Groups IA, IIA and IIIA. In general, the most suitableelements are lithium, magnesium, calcium, titanium, chromium, iron,nickel and aluminum. The promoters may be used in their elemental forme.g. as finely-divided or powdered metals, or they may be employed ascompounds of various types, both organic and inorganic, which areeffective to introduce the element into the reaction system, such asoxides, hydroxides, halides, e.g. bromides and iodides, oxyhalides,hydrides, alkoxides, and the like. Especially preferred organiccompounds are the salts of organic monocarboxylic acids e.g. alkanoatessuch as acetates, butyrates, decanoates and laurates, benzoates, and thelike. Other compounds include the metal alkyls, carbonyl compounds aswell as chelates, association compounds and enol salts. Particularlypreferred are the elemental forms, compounds which are bromides oriodides, and organic salts e.g. salts of the mono-carboxylic acidcorresponding to the anhydride being produced. Mixtures of promoters canbe used, if desired, especially mixtures of elements from differentGroups of the Periodic Table.

The quantity of the promoter can vary widely, but preferably it is usedin the amount of 0.001 mol to 100 mols per mol of Group VIII noble metalcatalyst, most preferably 0.001 to 10 mols per mol of catalyst.

In the separation of the products from the reaction mixtures, thepromoter generally remains with the Group VIII noble metal catalyst,i.e. as one of the least volatile components, and is suitably recycledor otherwise handled along with the catalyst.

Organic promoters capable of forming a coordination compound with theGroup VIII noble metal catalyst are beneficial, particularly organicnon-hydrocarbon materials containing within their molecular structureone or more electron rich atoms having one or more pairs of electronsavailable for formation of coordinate bonds with the noble metalcatalyst. Most such organic promoters can be characterized as Lewisbases for the particular anhydrous reaction system involved.

Suitable organic promoters are non-hydrocarbon materials capable offorming a coordination compound with the Group VIII noble metalcatalyst,, containing within their molecular structure one or more pairsof electrons available for formation of coordinate bonds with the noblemetal catalyst. Such promoters can be introduced concurrently with thereactants to the reaction zone or can be incorporated together with theGroup VIII noble metal by formation of ligand complexes with the noblemetal prior to introduction of the noble metal-ligand complex to thereaction zone.

Suitable organic promoters are organo-phosphine, organo-arsine,organo-stibine, organo-nitrogen, and organo-oxygen containing compounds.Organo-phosphine and organo-nitrogen promoters are preferred classes.

Suitable oxygen-containing compounds capable of functioning as organicpromoters in this system are those containing functional groups such asthe phenolic hydroxyl, carboxyl, carbonyloxy and carbonyl groups.Suitable organo-nitrogen containing compounds are those containingamino, imino and nitrilo groups. Materials containing both oxygen andnitrogen atoms can be used.

Illustrative organic promoters of the types mentioned above may be foundin British Pat. No. 1,538,782.

The quality of organic promoter employed is related to the quantity ofnoble metal catalyst within the reaction zone. Normally the quantity issuch that at least 0.1,, desirably at least 0.2, and preferably at least0.3 mol of promoter compound per mol of noble metal is present in thereaction zone. Preferably less than 100 mols of promoter per mol ofnoble metal catalyst would be used.

Carbon monoxide and hydrogen are preferably employed in substantiallypure form, as available commercially. However, inert diluents such ascarbon dioxide, nitrogen, methane, and/or inert gases (e.g., helium,argon, neon, etc.) can be present.

All reactants should be substantially free from water since, in thisfashion, the maintenance of a substantially anhydrous condition withinthe reaction zone is facilitated. The presence of minor amounts ofwater, however, such as may be found in these commercially availablereactants, is permissible. Normally, however, the presence of more than5 mol % of water in any one or more of the reactants should be avoided,the presence of less than 3 mol % of water is desired, and the presenceof less than 1.0 mol % of water is preferred. More important, however,than the amount of water in feed or recycle streams introduced to thereaction zone is the concentration of free water plus alcoholic hydroxylgroups (which react in situ to form water) present within the reactionzone. In practice, the molar ratio of (a) water plus the molarequivalents of alcoholic hydroxyl groups to (b) the number of mols ofdimethyl ether and/or methyl acetate within the reaction zone is themost convenient method for defining this concentration. On this basis,this ratio preferably should not exceed 0.1:1. Still lower values forthis ratio are advantageous, with optimal results being obtained withvalues for this ratio ranging from zero to 0.05:1.

Solvents or diluents can be employed, preferably materials which areindigenous to the reaction system and/or co-products commonly found inthe reaction system. Excess dimethyl ether and/or methyl acetate are thepreferred reaction diluents, with acetic acid being the preferredalternate. It is also practicable to employ organic solvents or diluentswhich are inert in the environment of the process. The most suitableinert solvents or diluents are hydrocarbons free from olefinicunsaturation, typically the paraffinic, cycloparaffinic, and aromatichydrocarbons such as octane, cyclododecane, benzene, toluene, and thexylenes. Other suitable solvents include chloroform, carbontetrachloride, and acetone.

The reactions require the presence of a halide, which would be acomponent of the liquid phase reaction medium. Suitable halides areeither bromide or iodide or mixtures thereof, iodide being preferred.The halide would usually be present largely in the form of methylhalide, acetyl halide, hydrogen halide, or mixtures of the foregoingspecies, and could be introduced to the liquid phase reaction medium assuch. However, these materials may be formed in situ, by using inorganichalide materials, e.g., salts such as the alkali metal and alkalineearth metal salts, as well as elemental iodine and bromine. Incontinuous operation, wherein reaction by-products are separated andrecycled to the reaction medium, organic halides such as methyl halidewill be present as components of the liquid phase reaction medium andcan be recovered and recycled to the reaction zone as such; thus, only asmall quantity of make-up halide need be supplied to compensate for suchlosses in recovery as may be encountered.

The amount of halide that should be present in the liquid phase reactionmedium is related to the amount of ether and/or ester reactantintroduced to the reaction zone, but otherwise can vary over a widerange. Typically, 0.5 to 1,000 mols of ester and/or ether per equivalentof halide, desirably 1 to 300 mols per equivalent, and preferably 2 to100 mols per equivalent are used. In general, higher proportions ofhalide to ether and/or ester reactant tend to increase reaction rate.

It has been found that molar ratios of carbon monoxide to hydrogen,broadly within the range of 1:100 to 100:1, desirably within the rangeof 50:1 to 1:50, and preferably within the range of 10:1 to 1:10 can beemployed. Best results are obtained with carbon monoxide-hydrogenmixtures which approach the stoichiometric ratios of carbon monoxide tohydrogen. Molar ratios of carbon monoxide to hydrogen within the rangeof 0.5:1 to 5:1 are thus especially preferred.

Recovery of Noble Metals

The invention broadly relates to the recovery of Group VIII noblemetals, typically rhodium, which appear to be bound to heavyhigh-boiling residues produced by carbonylation reactions, with orwithout hydrogen being present. Residues from such reactions are complexand have not been definitely analyzed. Generally, they are known tocontain high molecular weight compounds with organic carbonyl andacetate functions. They contain typically about 0-4 percent by weightrhodium after the volatile constituents have been removed. It ischaracteristic of such residues that they cannot be freed of all therhodium (or other noble metal) by extraction with halogen acids, such asdisclosed in the prior art.

The following example will illustrate a carbonylation process whichproduces heavy residues containing substantial amounts of rhodium.

EXAMPLE I

A one liter autoclave was operated continuously to produce aceticanhydride by the carbonylation of methyl acetate. The reactants, i.e.methyl acetate, methyl iodide, carbon monoxide and hydrogen are addedcontinuously, while the product acetic anhydride is removed as a vapor,condensed, and separated from the non-condensibles, which are returnedto the reactor. The reaction is catalyzed by the mixture of rhodiumtrichloride trihydrate, and lithium iodide, which are added to theinitial charge placed in the autoclave in amounts sufficient to provideabout 0.01 mol Rh/liter of liquid in the vessel and 60 mol Li/mol Rh.The reaction is operated at about 190° C., 50 kg/cm² absolute, withpartial pressures of about 40 kg/cm² CO and about 2.5 kg/cm² H₂. Theresidues are withdrawn at a rate sufficient to maintain a desiredconcentration in the autoclave and treated to recover the rhodiumvalues. The reaction mixture after being freed of a portion of itsvolatile components contains about 6 wt % methyl iodide, 10 wt % methylacetate, 50 wt % acetic anhydride, a small amount of ethylidenediacetate, 15-20 wt % acetic acid, with about 15 wt % heavy residuescontaining catalyst.

It has been found that treatment of heavy residues with halogen acids,disclosed in some of the prior art to be useful for extracting rhodium,is not adequate for commercial application since the high cost ofrhodium makes nearly complete recovery essential. As will be seen inrelated commonly assigned applications, (Ser. Nos. 241,193 and 241,181)acid treatment alone is insufficient with heavy residues produced by thecarbonylation processes of which Example I is representative.

The reagents which may be used to free rhodium (or other noble metals)from carbonylation residues are generally classified as amines, andparticularly the primary aliphatic amines, especially n-propylaminewhich has physical characteristics suitable for treating the residues.For purposes of the invention, primary aliphatic amines are consideredto include any reagent molecule with that functionality, regardless ofother functions also present in the molecule, including examples such as1,2 ethane diamine, 2 hydroxy ethylamine, 2 phenethyl amine, 2 chloroethylamine, 1,6 hexane diamine, and 4 amino-1-butene. Also, hydrazine ispreferred. Although it is not normally considered an amine, hydrazinehas the characteristic properties of an amine and for purposes of thisinvention is to classified.

The contacting of the reagent with the carbonylation residue is carriedout by removing as much of the carbonylation reaction mixture aspossible to leave concentrated residues, then mixing the residue withone or more of the selected treating agents, at a temperature in therange of 20°-200° C., a pressure typically about one atmosphere for atleast about one-half hour. Preferably, the amount of treating reagent(s)used will be at least one mol for each mol of carbonyl in the residue.

After the residue has been treated, it is contacted with a halogen acid,such as HCl, HI, or HBr. The acid may be added in various forms;typically an aqueous solution of about 10 wt % HCl or HI would be used.Generally, the acid will be introduced to the treated residue along witha solvent which can dissolve the residue and separate it from theaqueous layer which forms and which contains the extracted noble metal.While methylene chloride is used in the following examples, othersolvents could be used such as the indigenous compounds methyl iodideand methyl acetate (although others could be used). The extraction maybe carried out at room temperature, until the extraction is complete.Thereafter, the mixture is separated, with the residue being removed inthe solvent for disposal and the aqueous layer processed to recover thenoble metal content, reprocessed to form the catalyst, or recycleddirectly to the carbonylation reaction.

EXAMPLE II Comparative Example

Two samples (A & B) of the residue withdrawn in Example I are held attemperatures of up to 150° C. and about 760 mm Hg absolute pressure(Sample A) and at 50° C. and 0-5 mm Hg absolute pressure (Sample B) forabout 2 hours to evaporate volatile components and to leave behind onlythe heavy residues. The reduced residue contains about 1 wgt % rhodiumas measured by atomic absorption spectroscopy. It is intimately mixedwith 10 ml of a 3 N solution of HCl and 5 ml of methylene chloride atroom temperature which extracts as much rhodium as possible. Aftermixing is stopped, two layers form, a methylene chloride layercontaining the residue and an acid layer. No rhodium was removed by acidextraction in Sample A, but 32% of the rhodium was extracted in SampleB. The remainder of the rhodium is considered bound to the residue.

Since only part of the rhodium content is extracted by the acid, theremainder of must be recovered if intolerable losses of rhodium are tobe avoided. This may be accomplished by further treating the alreadyacid-extracted residues with reagents to free free the rhodium from theresidues, permitting a second acid extraction to successfully separatethe rhodium values.

The two samples of acid extracted residues are treated with a largeexcess of various reagents and then subjected to a second acidextraction. About 40-60 mg of the acid-treated residues in each sampleis heated to about 110° C. in the presence of the reagent being testedfor a period of 1-24 hours. Then, about 1 ml of 36 wt % aqueous HCl isadded plus 3 ml of 3 N HCl, along with 3 ml of methylene chloride. Aftermixing, the acid and methylene chloride layers are permitted to separateand the acid layer is removed. Another 3 ml of 3 N HCl is added torepeat the extraction step. After separating the second acid layer, theextraction is repeated for a third time by adding another 3 ml of 3 NHCl. The acid layers from the three extractions are mixed and analyzedfor rhodium. The results of a series of experiments carried out asdescribed are reported in the following table.

                  TABLE 1                                                         ______________________________________                                                              % bound Rho-                                                                  dium Extracted                                                                  Sample                                                Treating Agent          A       Sample B                                      ______________________________________                                        1 ml  n-butylamine          1.9     21.4                                      1/2 ml                                                                              aniline               1.0     8.6                                       1/2 ml                                                                              1-propylamine         <1      72.3                                      42 mg hydroxyl amine hydrochloride +                                                                      2.7     13.8                                            1 ml methylene chloride                                                 33 mg hydroxyl amine hydrochloride +                                                                      3.1     48.2                                            1 ml n-butyl amine                                                      --    2,4 dinitrophenyl hydrazine +                                                                       2.4     5.7                                             1/2 ml methylene chloride                                               0.2 ml                                                                              hydrazine monohydrate + 1 ml                                                                        2.7     78                                              methylene chloride                                                      0.4 ml                                                                              phenyl hydrazine + 1 ml methylene                                                                   <2.7    <6.6                                            chloride                                                                ______________________________________                                    

As the data in Table 1 shows, the vaporation of volatile materials atthe higher temperatures of Sample A causes the rhodium content to bemuch less easily extracted by an aqueous halogen acid, even aftertreatment with an amine or hydrazine.

It has been discovered that preparation of the residues by contactingwith an alkanol, preferably methanol, followed by evaporation at lowtemperatures, preferably not above about 25° C. An appropriate vacuumwould be used, typically between 1 and 10 mm Hg absolute. The benefitsof this preparation technique will be seen in the following example.

EXAMPLE III

A 41.5 gm portion of the residue of Example I is combined with 100 gmsof methanol and shaken overnight. The mixture is then evaporated in aBuchi Rotavapor R rotary evaporator at room temperature (about 25° C.)under a vacuum of about 35 mm Hg absolute overnight to evaporatevolatile compounds. The reduced material is 24% by weight of theoriginal sample and contains about 0.34 wt% rhodium, as measured byatomic absorption spectroscopy. A 52.2 mg portion of the solids istreated with 0.5 ml of n-propyl amine at 100° C. for one hour. Then 1 mlof 35 wt % aqueous HCl and 4 ml of methylene chloride are added and themixture extracted sequentially with three separate 3 ml portions of 3 NHCl, as in the previous examples. Analysis of the combined acid layersindicates that 96% of the rhodium had been extracted.

As shown in Example III and the Comparative Example II, the method ofremoving the volatile materials from carbonylation residues affects theability of the amine (or hydrazine) treatment to free rhodium from theresidues, permitting its subsequent extraction by an aqueous halogenacid. In the process of the invention, the carbonylation residue iscontacted with at least one mol of an alkanol, preferably methanol, foreach equivalent of acid ester and anhydride present in the heavy residuefor a sufficient period of time, at least about one hour, and then themixture is vaporized at a low temperature, preferably no higher than 25°C., and under a suitable vacuum, typically in the range of 1-40 mm Hgabsolute. The concentrated residues are then especially susceptable totreatment and extraction, permitting substantially complete recovery ofthe rhodium content.

What is claimed is:
 1. In a process for recovering Group VIII noblemetals bound to residues of noble metal catalyzed carbonylationreactions comprising separating said residues, vaporizing the volatilecomponents therefrom to produce a reduced residue, treating the reducedresidues with an amine, and extracting said noble metals from saidtreated reduced residue with an aqueous halogen acid, the improvementcomprising treating said residues with an alkanol or mixture thereof andthereafter vaporizing the volatile components at low temperatures and asuitable vacuum, thereby improving the subsequent recovery of said noblemetals.
 2. The process of claim 1 wherein said alkanol is methanol. 3.The process of claim 1 wherein said volatile components are removed at atemperature no higher than about 25° C.
 4. The process of claim 1wherein said vacuum is in the range of 1-40 mm Hg absolute.
 5. Theprocess of claim 1 wherein said alkanol is employed in at least one molfor every equivalent of acid ester and anhydride present in theresidues.
 6. The process of claim 1 wherein said carbonylation reactionis the carbonylation of methyl acetate or dimethyl ether to aceticanhydride.
 7. The process of claim 1 wherein said carbonylation reactionis the carbonylation in the presence of hydrogen of methyl acetate ordimethyl ether to ethylidene diacetate.
 8. The process for recoveringGroup VIII noble metals bound to residues of noble metal catalyzedcarbonylation reactions comprising:(a) separating said residues from thecarbonylation reaction mixture; (b) treating said separated residue of(a) with an alkanol or mixture thereof; (c) vaporizing the volatilecomponents of said treated residues of (b) at low temperatures and undera sufficient vacuum and recovering a reduced fraction therefrom; (d)treating said reduced fraction of (c) with a reagent comprising an aminein an amount sufficient to free said noble metals; (e) extracting saidtreated solid fraction of (d) with an aqueous halogen acid, therebyrecovering the noble metals therefrom.
 9. The process of claim 8 whereinsaid alkanol is methanol.
 10. The process of claim 8 wherein saidvolatile components are removed at a temperature no higher than about25° C.
 11. The process of claim 8 wherein said vacuum is in the range of1-40 mm Hg absolute.
 12. The process of claim 8 wherein said alkanol isemployed in at least one mol for every equivalent of acid ester andanhydride present in the residues.
 13. The process of claim 8 whereinsaid carbonylation reaction is the carbonylation of methyl acetate ordimethyl ether to acetic anhydride.
 14. The process of claim 8 whereinsaid carbonylation reaction is the carbonylation in the presence ofhydrogen of methyl acetate or dimethyl ether to ethylidene diacetate.15. The process of claim 8 wherein said amine reagent is at least onemember of the group consisting of primary aliphatic amines andhydrazine.
 16. The process of claim 8 wherein said Group VIII noblemetal is rhodium.